1. Field of the Invention
This invention relates to a radiation image processing method utilizing a neural network, wherein a subdivision pattern of radiation images, the shape and location of an irradiation field, an orientation in which an object was placed when the image of the object was recorded, a portion of an object of which the image was recorded, or the like, is determined from an image signal representing a radiation image by utilizing a neural network, and/or a filtering process, such as emphasis or correction, is carried out on an image signal representing a radiation image by utilizing a neural network.
The term "processing" as used hereinbelow means the determination of various items and the filtering process described above.
2. Description of the Prior Art
Techniques for reading out a recorded radiation image in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. For example, as disclosed in Japanese Patent Publication No. 61(1986)-5193, an X-ray image is recorded on an X-ray film having a small gamma value chosen according to the type of image processing to be carried out, the X-ray image is read out from the X-ray film and converted into an electric signal (image signal), and the image signal is processed and then used for reproducing the X-ray image as a visible image on a copy photograph, or the like. In this manner, a visible image, having good image quality with high contrast, high sharpness, high graininess, or the like, can be reproduced.
Also, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318, 4,387,428, and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object, such as the human body. A radiation image of the object is thereby stored on the stimulable phosphor sheet. The stimulable phosphor sheet is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then used during the reproduction of the radiation image of the object as a visible image on a recording material such as photographic film, on a display device such as a cathode ray tube (CRT) display device, or the like.
Radiation image recording and reproducing systems, which use stimulable phosphor sheets, are advantageous over conventional radiography using silver halide photographic materials, in that images can be recorded even when the energy intensity of the radiation, to which the stimulable phosphor sheet is exposed, varies over a wide range. More specifically, since the amount of light which the stimulable phosphor sheet emits when being stimulated varies over a wide range and is proportional to the amount of energy stored thereon during its exposure to the radiation, it is possible to obtain an image having a desirable density regardless of the energy intensity of the radiation to which the stimulable phosphor sheet was exposed. In order to obtain the desired image density, an appropriate read-out gain is set when the emitted light is being detected and converted into an electric signal to be used in the reproduction of a visible image on a recording material, such as photographic film, or on a display device, such as a CRT display device.
In order for an image signal to be detected accurately, certain factors which affect the image signal must be set in accordance with the dose of radiation delivered to the stimulable phosphor sheet and the like. Novel radiation image recording and reproducing systems which accurately detect an image signal have been proposed. The proposed radiation image recording and reproducing systems are constituted such that a preliminary read-out operation (hereinafter simply referred to as the "preliminary readout") is carried out in order to approximately ascertain the radiation image stored on the stimulable phosphor sheet. In the preliminary readout, the stimulable phosphor sheet is scanned with a light beam having a comparatively low energy level, and a preliminary read-out image signal obtained during the preliminary readout is analyzed. Thereafter, a final read-out operation (hereinafter simply referred to as the "final readout") is carried out to obtain the image signal, which is to be used during the reproduction of a visible image. In the final readout, the stimulable phosphor sheet is scanned with a light beam having an energy level higher than the energy level of the light beam used in the preliminary readout, and the radiation image is read out with the factors affecting the image signal adjusted to appropriate values on the basis of the results of an analysis of the preliminary read-out image signal.
The term "read-out conditions", as used hereinafter, means a group of various factors, which are adjustable and which affect the relationship between the amount of light emitted by the stimulable phosphor sheet during image readout and the output of a read-out means. For example, the term "read-out conditions" may refer to a read-out gain and a scale factor which define the relationship between the input to the read-out means and the output therefrom, or to the power of the stimulating rays used when the radiation image is read out.
The term "energy level of a light beam", as used herein, means the level of energy of the light beam to which the stimulable phosphor sheet is exposed per unit area. In cases where the energy of the light emitted by the stimulable phosphor sheet depends on the wavelength of the irradiated light beam, i.e. the sensitivity of the stimulable phosphor sheet to the irradiated light beam depends upon the wavelength of the irradiated light beam, the term "energy level of a light beam" means the weighted energy level which is calculated by weighting the energy level of the light beam, to which the stimulable phosphor sheet is exposed per unit area, with the sensitivity of the stimulable phosphor sheet to the wavelength. In order to change the energy level of a light beam, light beams of different wavelengths may be used, the intensity of the light beam produced by a laser beam source or the like may be changed, or the intensity of the light beam may be changed by moving an ND filter or the like into and out of the optical path of the light beam. Alternatively, the diameter of the light beam may be changed in order to alter the scanning density, or the speed at which the stimulable phosphor sheet is scanned with the light beam may be changed.
Regardless of whether the preliminary readout is or is not carried out, it has also been proposed to analyze the image signal (or the preliminary read-out image signal) obtained and to adjust the image processing conditions, which are to be used when the image signal is processed, on the basis of the results of an analysis of the image signal. The term "image processing conditions", as used herein, means a group of various factors, which are adjustable and set when an image signal is subjected to processing, which affects the gradation, sensitivity, or the like, of a visible image reproduced from the image signal. The proposed method is applicable to cases where an image signal is obtained from a radiation image recorded on a recording medium such as conventional X-ray film, as well as to systems using stimulable phosphor sheets.
As disclosed in, for example, Japanese Unexamined Patent Publication No. 61(1986)-280163 and U.S. Pat. No. 4,638,162, operations for calculating the values of the read-out conditions for the final readout and/or the image processing conditions are carried out by a group of algorithms which analyze an image signal (or a preliminary read-out image signal). A large number of image signals detected from a large number of radiation images are statistically processed. The algorithms which calculate the read-out conditions for the final readout and/or the image processing conditions are designed on the basis of the results obtained from this processing.
In general, the algorithms which have heretofore been employed are designed such that a probability density function of an image signal is created, and characteristic values are found from the probability density function. The characteristic values include, for example, the maximum value of the image signal, the minimum value of the image signal, or the value of the image signal at which the probability density function is maximum, i.e. the value which occurs most frequently. The read-out conditions for the final readout and/or the image processing conditions are determined on the basis of the characteristic values.
However, in cases where the read-out conditions for the final readout and/or the image processing conditions are calculated on the basis of the results of an analysis of the image signal in the manner described above, and the image signal is detected from a recording medium, on which the irradiation field was limited during the recording of the radiation image, the radiation image cannot be ascertained accurately if the image signal is analyzed without the shape and location of the irradiation field being taken into consideration. As a result, incorrect read-out conditions and/or incorrect image processing conditions are set, and it becomes impossible to reproduce a visible radiation image which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness.
In order to eliminate the aforesaid problem, it is necessary to determine the shape and location of an irradiation field and then to calculate the read-out conditions for the final readout, and/or the image processing conditions, on the basis of only the image signal representing image information stored in the region inside of the irradiation field.
The applicant has proposed various methods for determining the shape and location of an irradiation field as disclosed in, for example, U.S. Pat. Nos. 4,967,079, 4,851,678 and 4,931,644. The aforesaid problems can be eliminated by automatically determining the shape and location of the irradiation field by use of the proposed methods, and setting the read-out conditions for the final readout, and/or the image processing conditions, only for the region inside of the irradiation field thus found.
When radiation images are recorded on recording media, subdivision image recording is often carried out. With the subdivision image recording, the recording region on a single recording medium is divided into a plurality of predetermined subdivisions, and the respective subdivisions are exposed to radiation for image recording. The subdivision image recording is economical since, for example, when images of small object portions are recorded on large recording media, images of a plurality of object portions can be recorded on a single recording medium. Also, the speed with which radiation images are recorded and read out can be kept high.
However, in cases where irradiation fields are limited when the subdivision image recording described above is carried out on a single recording medium, the respective irradiation fields become separated from each other on the recording medium. In such cases, the shapes and locations of the irradiation fields cannot be determined accurately. A method for automatically determining the shapes and locations of a plurality of irradiation fields on a single recording medium has also been proposed. However, with the proposed method, the algorithms for determining the shapes and locations of irradiation fields become very complicated, and a very expensive apparatus is necessary for carrying out the method.
If information concerning the positions of the respective subdivisions is given by manually entering the information representing a subdivision pattern on the recording medium into an apparatus for determining the shape and location of an irradiation field when the shapes and locations of the irradiation fields are to be detected, an operation for detecting a single irradiation field in each subdivision may be carried out. Therefore, the problems in which the algorithms for determining the shapes and locations of the irradiation fields become very complicated can be eliminated. However, considerable time and labor are required to enter the information concerning the subdivision pattern each time radiation images are read out from a recording medium.
Accordingly, a need exists for a method with which a subdivision pattern of radiation images recorded on a recording medium can be determined automatically. The applicant proposed various methods for automatically determining a subdivision pattern of radiation images which have been recorded on a recording medium in, for example, U.S. Pat. Nos. 4,829,181, 4,962,539, and 5,042,074, and Japanese Unexamined Patent Publication Nos. 2(1990)-267679, 2(1990)-272530, 2(1990)-275429, 2(1990)-275432, 2(1990)-275435 and 2(1990)-296235.
Also, in cases where the read-out conditions for the final readout, and/or the image processing conditions, are determined in the manner described above, it often occurs that, when radiation images of a single object were recorded on recording media with the object being placed in different orientations, the image density of a region of interest in the object varies for visible images reproduced from the radiation images.
For example, when the thoracic vertebrae of a human body are to be diagnosed, radiation images of the chest of the human body are recorded on stimulable phosphor sheets from the front and a side of the chest. In cases where the radiation image of the chest is recorded from the front of the chest, the thoracic vertebrae, which are the region of interest, overlap the mediastinum region, through which the radiation cannot pass easily. Therefore, in such cases, the amount of energy stored on part of the stimulable phosphor sheet corresponding to the thoracic vertebrae during its exposure to the radiation is comparatively small. As a result, when the stimulable phosphor sheet, on which the frontal chest image has been stored, is exposed to stimulating rays during the operation for reading out the radiation image, the part of the stimulable phosphor sheet corresponding to the thoracic vertebrae emits a comparatively small amount of light. On the other hand, in cases where the radiation image of the chest is recorded from the side of the chest, the thoracic vertebrae, which are the region of interest, overlap the lung fields, through which the radiation can pass easily. Therefore, in such cases, the amount of energy stored on part of the stimulable phosphor sheet corresponding to the thoracic vertebrae during its exposure to the radiation is comparatively large. As a result, when the stimulable phosphor sheet, on which the lateral chest image has been stored, is exposed to stimulating rays during the operation for reading out the radiation image, the part of the stimulable phosphor sheet corresponding to the thoracic vertebrae emits a comparatively large amount of light. Also, the maximum value and the minimum value of the image signal detected from the stimulable phosphor sheet do not vary much for the frontal chest image and the lateral chest image. Therefore, when the read-out conditions for the final readout, and/or the image processing conditions, are adjusted with conventional techniques in accordance with the maximum value and the minimum value of the image signal detected from the stimulable phosphor sheet, approximately the same values of the read-out conditions for the final readout, and/or approximately the same values of the image processing conditions, are set for the frontal chest image and the lateral chest image. Accordingly, if image signals are detected from the frontal chest image and the lateral chest image under the thus set read-out conditions for the final readout, and visible images are reproduced from the detected image signals, and/or if the image signals detected from the frontal chest image and the lateral chest image are processed under the thus set image processing conditions, and visible images are reproduced from the processed image signals, the image density of the patterns of the thoracic vertebrae in the visible image reproduced from the frontal chest image becomes comparatively low, and the image density of the patterns of the thoracic vertebrae in the visible image reproduced from the lateral chest image becomes comparatively high.
In order for the aforesaid problems to be eliminated, information concerning in what orientation the object was placed when the image of the object was recorded has heretofore been entered into an image read-out apparatus or an image processing unit each time the radiation image is read out from a stimulable phosphor sheet. The read-out conditions for the final readout, and/or the image processing conditions, are set in accordance with the entered information concerning the orientation in which the object was placed when the image of the object was recorded.
However, considerable time and labor are required to enter the information concerning the orientation, in which the object was placed when the image of the object was recorded, each time a radiation image is read out from a stimulable phosphor sheet. Also, it will easily occur that incorrect information concerning the orientation, in which the object was placed when the image of the object was recorded, is entered.
Therefore, a method for automatically determining the orientation, in which the object was placed when the medical image of the object was recorded on a stimulable phosphor sheet, or the like, has been proposed in, for example, U.S. Pat. No. 4,903,310.
Additionally, for the same reasons as those described above, in cases where the read-out conditions for the final readout, and/or the image processing conditions, are determined in the manner described above, it often occurs that the image density of a region of interest in an object varies in a reproduced visible image, depending on which portion of the object was recorded (e.g., the head, the neck, the chest, or the abdomen in cases where the object is a human body). In order for such problems to be eliminated, information concerning which portion of the object was recorded has heretofore been entered into an image read-out apparatus or an image processing unit each time the radiation image is read out from a stimulable phosphor sheet. The read-out conditions for the final readout, and/or the image processing conditions, are set in accordance with the entered information concerning the portion of the object of which an image was recorded.
As described above, when the read-out conditions for the final readout, and/or the image processing conditions, are set, it is necessary to determine a subdivision pattern of radiation images, the shape and location of an irradiation field, an orientation in which an object was placed when the image of the object was recorded, a portion of an object of which an image was recorded, or the like. A correction should then be made in accordance with the results of the determination. Thereafter, appropriate read-out conditions for the final readout, and/or appropriate image processing conditions, should be set.
With the aforesaid methods for determining the shape and location of an irradiation field, in cases where the contour (i.e. the edge) of the irradiation field is unsharp, the shape and location of the irradiation field cannot be determined accurately.
Also, even if an irradiation field is limited on a recording medium by using a mask, scattered radiation will turn to the side of the mask and will impinge upon the region outside of the irradiation field. As a result, density fog occurs in the region outside of the irradiation field on the recording medium. In such cases, the shape and location of the irradiation field cannot be determined accurately with the aforesaid methods for determining the shape and location of an irradiation field.
In order for the problems described above to be solved, before the shape and location of an irradiation field are determined, image emphasis may be carried out such that the edge of the irradiation field may be emphasized and adverse effects of the scattered radiation in the region outside of the irradiation field may be eliminated. Also, in cases where a remark portion is present at part of a radiation image, a filtering process may be carried out only on image signal components representing image information stored at said part of the radiation image in order to emphasize said part of the radiation image.
Additionally, in cases where defects, such as missing points, are present in an image signal which has been obtained from signal compression processing, or the like, an image correction utilizing a filtering process is carried out on the image signal in order to eliminate the defects.
Further, an image correction utilizing a filtering process, such as an image smoothing process or an image noise eliminating process, is carried out on an image signal representing a radiation image in order to improve the image quality of the radiation image.
As described above, during image processing on a radiation image, various filtering processes, such as emphasis and correction, are carried out on the image signal representing the radiation image such that an appropriate visible image may be reproduced.
Recently, a method for utilizing a neural network has been proposed and is being used in various fields.
The neural network is provided with a learning function by a back propagation method. Specifically, when information (an instructor signal), which represents whether an output signal obtained when an input signal is given is or is not correct, is fed into the neural network, the weights of connections between units in the neural network (i.e. the weights of synapse connections) are corrected. By repeating the learning operation of the neural network, the probability that a correct answer will be obtained in response to a new input signal can be kept high. (Such functions are described in, for example, "Learning representations by back-propagating errors" by D. E. Rumelhart, G. E. Hinton and R. J. Williams, Nature, 323-9,533-536, 1986a; "Back-propagation" by Hideki Aso, Computrol, No. 24, pp. 53-60; and "Neural Computer" by Kazuyuki Aihara, the publishing bureau of Tokyo Denki University).
By feeding an image signal representing a radiation image into the neural network, the determination of various items and various filtering processes described above can be carried out with the neural network. A correction can then be made in accordance with the results of the processing. Thereafter, appropriate read-out conditions for the final readout and/or appropriate image processing conditions can be set.
Specifically, when the determination of various items and various filtering processes are to be carried out, an image signal representing a radiation image is fed into the neural network. From the neural network, outputs representing characteristic measures, which indicate the results of the processing, are obtained. By repeating the learning operation of the neural network, outputs more accurately representing the characteristic measures can be obtained.
When the determination of various items, such as a subdivision pattern of radiation images, the shape and location of an irradiation field, an orientation in which an object was placed when the image of the object was recorded, and a portion of an object, the image of which was recorded, and/or a filtering process, such as emphasis or correction, is carried out by utilizing a neural network, outputs representing the characteristic measures, which indicate the results of the determination and/or the results of the filtering process, can be obtained from the neural network only by feeding an image signal, made up of a series of image signal components representing the picture elements in the radiation image, into the neural network. Therefore, various types of accurate processing can be carried out easily.
The applicant has proposed several methods for carrying out the determination of various items and a filtering process by utilizing a neural network. Such methods are proposed in, for example, U.S. patent application Ser. No. 687,140, now abandoned, and Japanese Patent Application No. 3(1991)-76534.
In Japanese Patent Application No. 3(1991)-76534, and U.S. patent application Ser. No. 687,140, now abandoned, an overall connection type of neural network is described.
Specifically, in cases where the neural network is composed of an input layer (a first layer), an intermediate layer (a second layer), and an output layer (a third layer), the image signal components corresponding to the whole region of an original image are fed into each of the neurons of the intermediate layer (i.e. the second layer). Such neural network is herein referred to as the overall connection type of neural network.
The learning operation of a neural network is carried out with respect to a given subject such that the difference between the output and a correct answer becomes smallest. In the overall connection type of neural network described above, the degree of freedom of the learning operation becomes very high. Therefore, depending on the set values of the initial conditions, the output of the neural network does not converge to the value resulting from the stable state of the neural network, i.e. to the global minimum, which is the minimum value of energy E. But instead, the output of the neural network is trapped at the local minimum of a shallow concave part, at which energy E locally takes the minimum value.
In general, the operation means of the neural network takes on the form of a black box, and no person can intervene in the operation means. Therefore, if the output of the neural network is trapped at the local minimum, a value, which is markedly different from the correct answer, is obtained as the output from the neural network.